The present invention relates to subterranean treatment fluids. In particular, the present invention relates to subterranean treatment fluid compositions that comprise polyoxazoline compositions and methods for using such compositions in subterranean applications.
A subterranean treatment fluid may be used in a subterranean formation in a variety of ways. For example, a treatment fluid may be used to drill a borehole in a subterranean formation, to stimulate a well bore in a subterranean formation, or to clean up a well bore in a subterranean formation, as well as for numerous other purposes. As used herein, “treatment fluid” refers to any fluid that may be used in a subterranean application in conjunction with a desired function and/or for a desired purpose. The term “treatment fluid” does not imply any particular action by the fluid.
A drilling fluid (one type of treatment fluid) often used in connection with drilling a well bore in a subterranean formation can be any number of fluids (gaseous or liquid) and mixtures of fluids and solids (such as solid suspensions, mixtures and emulsions of liquids, gases and solids) used in operations to drill well bores into subterranean formations. Drilling fluids are used, inter alia, to cool the drill bit, lubricate the rotating drill pipe to prevent it from sticking to the walls of the well bore, prevent blowouts by serving as a hydrostatic head to counteract the sudden entrance into the well bore of high pressure formation fluids, and remove drill cuttings from the well bore.
During drilling of subterranean well bores, it is common to encounter portions of the subterranean formation that contain materials that may react undesirably with water, e.g., shales or clays. For convenience, as referred to herein, the term “shale” shall be understood to include any subterranean materials that may “swell,” or increase in volume, when exposed to water, whether commonly referred to as shale, clay, or other some subterranean material. Shales may be problematic during drilling operations, inter alia, because of their tendency to become chemically and/or physically altered when exposed to aqueous media such as aqueous-based drilling fluids. This alteration, of which swelling is one example, can result in undesirable drilling conditions and undesirable interference with the drilling fluid. For instance, the degradation of the shale may interfere with attempts to maintain the integrity of drilled cuttings traveling up the well bore until such time as the cuttings can be removed by solids control equipment located at the surface. Degradation, or erosion, of drilled cuttings prior to their removal at the surface can prolong drilling time, because shale particles traveling up the well bore may break into smaller and smaller particles, which increasingly exposes more of the shale's surface area to the drilling fluid, which leads to still further absorption of water, and further degradation.
Shale disintegration may also adversely impact “equivalent circulation density” (ECD). ECD is affected by the solids content of the drilling fluid, which usually increases if surface solids control equipment cannot remove shale from the drilling fluid. Plastic viscosity (an indicator of size and quantity of solids) is an important parameter in determining drilling rate. Maintenance of appropriate ECD is important, for example, in situations where a subterranean well bore is being drilled wherein a narrow tolerance exists between the weight of the drilling fluid needed to control the formation pressure, and the weight of the drilling fluid that will fracture the formation. In such circumstances, minimizing shale degradation provides improved control of the density of the drilling fluid, and enhances the probability of successfully drilling a well bore.
Shale degradation may substantially decrease the stability of the well bore, which may cause irregularities in the diameter of the well bore, e.g., the diameter of some portions of the well bore may be either smaller or greater than desired. In an extreme case, shale degradation may decrease the stability of the well bore to such an extent that the well bore will collapse, possibly in effect, inter alia, causing damage to the surrounding formation. Degradation of the shale may also, inter alia, interrupt circulation of the drilling fluid, cause greater friction between the drill string and the well bore, or cause the drill string to become stuck in the well bore. These and other complications that may be associated with shale swelling may greatly increase costs associated with subterranean operations.
A traditional method of inhibiting shale swelling during drilling to attempt to minimize such complications has been to use an oil-based drilling fluid as opposed to an aqueous-based drilling fluid. However, oil-based drilling fluids are often environmentally undesirable because they may be toxic to marine plants and animals. Accordingly, environmental regulations enacted by numerous countries have curtailed the use of oil-based drilling fluids. Consequently, water-based drilling fluids are preferred because they may have a more benign effect on the environment than oil-based drilling fluids.
Another means to counteract the propensity of aqueous drilling fluids to interact with reactive shales in the formation is to add a shale-inhibiting component to the aqueous drilling fluid. As referred to herein, the term “shale-inhibiting component” will be understood to mean a compound that demonstrates a propensity for inhibiting the tendency of a sample of shale to absorb water, often by adhering to the shale's surface and/or insertion between clay platelets. Commonly used shale-inhibiting components are polyacrylamides. Polyacrylamide shale-inhibiting components, however, are becoming more environmentally undesirable, especially in heavily regulated areas, because they generally demonstrate low biodegradability and high toxicity. Moreover, polyacrylamide shale-inhibiting components contain a toxic residual monomer (e.g., an unreacted monomer) that is undesirable. Additionally, the degradation products of polyacrylamide shale-inhibiting components are not useful; consequently, usually more must be added to keep the desired concentration at a level to achieve a desired effect (which means more of the undesirable residual monomer is added to the system). Potassium chloride is another material that has been utilized as a shale inhibitor, but it is considered to be only moderately effective at inhibiting the swelling of shale. Furthermore, potassium chloride is environmentally unacceptable in some areas of the world, e.g., the North Sea and the Gulf of Mexico. Potassium chloride also is disfavored in the Middle East, where wells may be drilled in close proximity to aquifers. Polyglycols also have been used as shale inhibitors in water-based drilling fluids but have not reached satisfactory inhibition levels. Partially hydrolyzed polyacrylamides also have been utilized in many regions, but these have been found to cause formation damage and generally are regarded as environmentally undesirable